When an epitaxial layer has been grown on a single crystal substrate by conventional vapor phase epitaxy, in particular, metal-organic chemical vapor deposition (MOCVD), convex defects or concave defects have sometimes generated on or in a surface of the epitaxial layer.
Convex defects are called tear-drop-like defects or hillocks; their diameters are 10 μm to 30 μm and their heights are several tens of nanometers. Despite the convex defects, semiconductor devices can be manufactured, but in some cases, the defects have damaged photo masks used in their manufacturing process or have caused displacements of patterns. Therefore, to reduce convex defects, a method of controlling off-angle of single crystal substrate have been proposed (for example, JP-A-2-239188 and JP-A-8-78348).
On the other hand, concave defects have diameters of several micrometers and depth reaching the vicinity of an interface between a single crystal substrate and an epitaxial layer. When a semiconductor device is manufactured by using a compound semiconductor epitaxial substrate with concave defects, the semiconductor device yield decreases.
Besides, it has been difficult to reduce the concave defects through the use of the method for reducing the convex defects.